In recent years, cellular mobile communications have advanced from UMTS (Universal Mobile Telecommunication System) to LTE (Long Term Evolution). In LTE, a method based on OFDM (Orthogonal Frequency Division Multiplexing) is specified as a wireless access technique, and high-speed wireless packet communications can be performed at a downlink peak transmission rate of 100 Mb/s or higher and at an uplink peak transmission rate of 50 Mb/s or higher. Furthermore, 3GPP (3rd Generation Partnership Project), which is an international standards organization, has started discussing LTE-A (LTE-Advanced) as a mobile communication system based on LTE, so as to realize even faster communications.
In LTE and LTE-A, DRX (Discontinuous Reception), which is a method for operating a mobile station in a power saving mode, may be used. DRX is now described in detail. In a mobile station, there are periods during which a monitoring of PDCCHs (Physical Downlink Data Channels) as L1 signals for controlling data transmission/reception is required, and there are periods during which the monitoring of PDCCHs is not required. Those periods are configured in cycles, and are called DRX cycles. Those cycles are not configured for each application, but are uniquely configured for each mobile station. In the periods during which PDCCHs is not required to be monitored, there is not required to perform signal processing such as data processing, and accordingly, a mobile station can enter a power saving mode. In the power saving mode, the mobile station suspends signal processing at its baseband unit, for example. The DRX cycles are configured by a base station notifying the mobile station of parameters or the like using an RRC (Radio Resource Control) signaling as an L3 signaling at the start of a communication. More specifically, the mobile station receives an RRC signaling that is a control signal, and configures DRX in accordance with DRX parameters transmitted through the RRC signaling. The start of DRX is specified by an SFN (System Frame Number) function. The other basic DRX parameters include OnDuration Timer. The period during which this timer is in operation is referred to as OnDuration, and a mobile station is unable to enter the power saving mode in this period. In a period during which this timer is not in operation, or in a period other than OnDuration, a mobile station can enter the power saving mode as specified.
Further, in LTE and LTE-A, two methods called DS (Dynamic Scheduling) and SPS (Semi-Persistent Scheduling) are specified as a data reception scheduling method. According to DS, radio resources being used by PDCCHs are indicated in both uplink and downlink communications, and PDSCHs (Physical Downlink Shared Channels) are transmitted as data with indicated parameters. According to SPS, on the other hand, a mobile station is notified of predetermined periods having predetermined transmission cycles before a communication is actually performed. When an SPS communication is performed, an activation command in a PDCCH is transmitted by the base station to notify the mobile station of which radio resources are continuously used, and the communication is then started. The base station also transmits a release command or the like in a PDCCH to the mobile station, to suspend a communication.
In LTE and LTE-A, blind decoding is performed on PDCCHs when PDCCHs are monitored. The base station assigns PDCCHs to radio resources called CCE (Control Channel Elements), and transmits the PDCCHs. The base station generates PDCCHs for each mobile station, and assigns the corresponding number of CCEs to the PDCCHs required in control information. The base station maps the control information defined by DCI (Downlink Control Information) formats on the physical resources corresponding to the assigned CCEs, and then transmits the control information. For example, in order to meet a reception quality requirement, an MCS (Modulation and Coding Scheme) having a low MCS level requires to be configured for a mobile station in a location with poor transmission channel quality such as a location near a cell boundary. In such a case, the base station transmits a PDCCH occupying a larger number of CCEs, such as eight CCEs. Meanwhile, even if an MCS having a high MCS level is configured for a mobile station in a location with excellent transmission channel quality such as a location near the center of a cell, it is possible to meet a reception quality requirement. Accordingly, the base station transmits a PDCCH occupying a smaller number of CCEs, such as one CCE. Further when mapping on physical resources is performed, mapping may be performed in regions unique to the cell or may be performed in regions unique to a mobile station, depending on the type of control information. Such regions in which PDCCHs are mapped on physical resources are called a search space.
The base station puts PDCCHs for respective mobile stations into one sub-frame, and simultaneously transmits the PDCCHs. Here, the number of CCEs occupied by the PDCCH for one mobile station is called an aggregation level, and the number of CCEs occupied by all PDCCHs is called an aggregation size. The base station does not notify mobile stations of which physical resources PDCCHs are mapped on. Therefore, each mobile station requires to determine whether a PDCCH directed to its own station is included in the sub-frame containing PDCCHs. This process is referred to as blind decoding. Specifically, a mobile station detects a PDCCH directed to its own station by detecting and decoding PDCCHs at each of the aggregation levels in each of the search spaces configure in the sub-frame. Such blind decoding is performed at the baseband unit of each mobile station, and therefore, the power of the mobile station is consumed. To reduce power consumption in the mobile station, it is preferable to reduce the number of times blind decoding is performed.
Meanwhile, there are an increasing number of mobile stations such as smartphones that execute a large number of applications. As intermittent traffics are generated from each application, there are traffics that characteristically appear to be continuous. Therefore, if a large number of applications intermittently generate data, the traffics appear to be continuous from the viewpoint of a mobile station. In a case where conventional DRX is used, a long OnDuration requires to be configured, and therefore, a mobile station is unable to enter the power saving mode. Further, in a mobile station, the periods for monitoring PDCCHs and the periods not for monitoring PDCCHs are configured simply in a regular pattern. Therefore, when PDCCHs are monitored in blind decoding, the maximum number of formats of PDCCHs are monitored. As a result, by a conventional method, monitoring of the maximum number of PDCCHs is continued over the periods during which the monitoring of PDCCHs is required.
In view of this, a technique for performing communications by switching between a communication using DS and a communication using SPS has been suggested as a conventional technique for efficiently assigning radio resources.    Patent Literature 1: Japanese Laid-open Patent Publication No. 2011-66639    Non-Patent Literature 1: 3GPP TR 36.913, “Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA) (LTE-Advanced)”, V9.0.0, Release 9, December 2009.    Non-Patent Literature 2: 3GPP TR36.912, “Feasibility study for further advancements for E-UTRA (LTE-Advanced)”, V9.3.0, Release 9, June 2010.    Non-Patent Literature 3: 3GPP TS36.321, “Medium Access Control (MAC) protocol specification”, V10.2.0, Release 10, June 2011.    Non-Patent Literature 4: 3GPP TS36.133, “Requirements for support of radio resource management”, V10.3.0, Release 10, June 2011.    Non-Patent Literature 5: 3GPP TS36.213, “Physical layer procedures”, V10.2.0, Release 10, June 2011.    Non-Patent Literature 6: 3GPP TS36.300, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)”, V10.4.0, Release 10, June 2011.